1. Field of the Invention
The present invention relates to a head IC, a read circuit, a media storage device and an amplifier gain adjustment method for a head IC which adjust output levels of read signals of an head, and more particularly to a head IC, read circuit, media storage device and amplifier gain adjustment method for a head IC which adjust the read signals of heads having different output levels to a predetermined level.
2. Description of the Related Art
In a media storage device, such as a magnetic disk device, a head reads data from media. Along with an increase in track density in recent years, heads having high sensitivity in detecting the magnetic flux on the media are being provided. For example, an MR (Magnetic Resistance) element is used for a read head. On the other hand, the use of a TuMR (or TMR) element, which utilizes the tunnel effect, is under consideration as an element having an even higher detection sensitivity.
Such a read element has high detection sensitivity, so manufacturing the elements having uniform characteristics is difficult. Hence the output level of an individual read element is not constant, and also changes depending on the characteristic change and fluctuation of the floating amount of the element due to temperature fluctuation. Therefore a circuit to adjust the output level of the read element is required before demodulating data.
FIG. 11 is a diagram depicting a configuration of a conventional media storage device, and FIG. 12 is a block diagram of a conventional read circuit. As FIG. 11 shows, magnetic heads 202a and 202b, to read data on a media (magnetic disk) 210, are disposed at the tip of an actuator 200.
The actuator 200 rotates around a rotation axis 204 and positions the magnetic heads 202a and 202b on a desired track of the magnetic disk 210. The magnetic heads 202a and 202b are connected to a head IC 220 provided on the actuator 200, and the head IC 220 is connected to a control circuit 230 outside the actuator 200.
Since the distances from the magnetic heads 202a and 202b to the control circuit 230 are long, the signal levels of the magnetic heads 202a and 202b may change, and noise may enter, so the head IC 220 is disposed between the magnetic heads 202a and 202b and the control circuit 230 and adjusts the signal level. This head IC 220 also plays a part of integrating many signal lines between the magnetic head and the control circuit to simplify wiring. For example, the control circuit 230 and the head IC 220 are connected via a serial interface (SI), and the head IC 220 and each magnetic head 202a and 202b are connected with individual signal lines.
FIG. 12 is a detailed diagram of a read system circuit in FIG. 11, where a variable gain amplifier (preamplifier) 220-1 is disposed in the head IC 220, which is connected to a read element 202-1 of the magnetic head 202a (or 202b), and amplifies the output of the read element 202 at a gain which is set. In the variable gain amplifier 220-1, the number of settings in the gain adjustment range is four, and gain is set and adjusted for each head before the device is shipped.
In the control circuit 230, which is connected to the head IC 220, a read channel 230-1 is disposed, and an AGC (Automatic Gain Control) amplifier 230-2, disposed in the read channel 230-1, receives output from the variable gain amplifier 220-1 of the head IC 220.
The AGC amplifier 230-2 is comprised of a differential amplifier 230-3 and an AGC circuit 230-4. The AGC circuit 230-4 compares an output value of the differential amplifier 230-3 and a reference output value, feeds back the comparison result to adjust the gain of the differential amplifier 230-3, and adjusts the output level of the differential amplifier 230-3 to the reference level (e.g. see Japanese Patent Application Laid-Open No. H10-021647 (FIG. 1 and others)).
The AGC circuit 230-4 sets a high-speed feedback coefficient (frequency, gain change amount) which follows up the change in one sector of a track of the magnetic disk 210, and equalizes the signal level within one sector.
In the prior art, the fluctuation of the output level caused by the characteristics of the element itself and the fluctuation, due to environmental conditions, including the floating amount change, are mainly adjusted by the AGC amplifier of the read channel.
However use of the TuMR element, which has a higher detection sensitivity than this read element, is desired, due to the recent increase in recording density. This read element having a high detection sensitivity is about 10 times that of an MR element, but dispersion of the individual signal output level is also high accordingly.
Also the fluctuation of the signal level, due to the fluctuation of temperature and floating amount, is also high because the detection sensitivity is high. And it may be difficult that the AGC amplifier in the read channel adjusts such major fluctuations of the signal level.
Also because of the increase in recording density, circuit frequency has increased, and a high-speed AGC amplifier of the read channel is also demanded. Such AGC amplifier has a narrower input dynamic range of the AGC. As a result, it is difficult to adjust the fluctuation of the level by changing the AGC amplifier of the read channel.
It could be possible to adjust the setting value of the variable gain amplifier, but adjusting the setting value of the variable gain amplifier is difficult since the number of steps are few, for example four. It could be possible to adjust the setting value of the variable gain amplifier by increasing the number of steps, but this spreads the distribution of the gain setting which is set initially, and adjustment before reading the system area of the disk takes time.
Also because increasing the number of steps requires considering relationships with the dynamic range of the AGC amplifier of the read channel, it is necessary to adjust even more time and to limit the adjustment of the level fluctuation.